If you've been in the market for a rental apartment recently, you may have noticed something strange.

The median U.S. price of a one-bedroom surpassed the price of a two-bedroom in December 2015, and the gap has been widening ever since, according to indexes created by online real estate company Zillow to track prices in hundreds of U.S. cities. The trend is being driven by prices in a specific set of metro areas.

It's not that two-bedrooms are getting cheaper — they are still appreciating at around the same 3 percent annual pace as the U.S. rental market overall. It's just that the median rent on a one-bedroom place has been growing much faster, at around 5 percent each year.

So if an American is willing to live anywhere, he or she could likely find a two-bedroom rental for less than a one-bedroom. Of course, if they split that apartment with a roommate, they could effectively cut their rent in half.

That national trend could have several explanations. In many cities, the newer, more expensive rentals in the trendiest parts of the metro area are often one-bedroom homes.

"Hip, new multifamily buildings are being built, and those are more likely to be one-bedroom homes," said Skylar Olsen, senior economist at Zillow. "Across the whole housing stock, it could be that one-bedrooms are more expensive because they're in more attractive parts of town."

Consider that there is a 1,000-square-foot, two-bedroom apartment in Brooklyn's Bedford-Stuyvesant neighborhood currently listed on Zillow for $2,300 a month.

"In places where land is more expensive, like downtown New York, that's where rent is exploding and appreciating faster in the tight urban areas."-Skylar Olsen, senior economist, Zillow

You could also choose a 450-square-foot, one-bedroom in Manhattan's West Village for $2,925 a month, or a 700-square-foot, one-bedroom in the trendy Brooklyn neighborhood of Williamsburg for $3,350 a month, according to current listings. In vast cities like New York, the price differences between neighborhoods can be huge, and a preference for one-bedrooms in expensive areas can have a big effect on the median overall.

Rents are also increasing faster in places that have more one-bedrooms already — like New York or San Francisco. Skyrocketing rents in those places lead to increases in the median cost of one-bedrooms nationwide.

"In the middle of the country, land is pretty cheap and space is pretty cheap, so you're more likely to have housing stock with two-bedrooms," said Olsen. "In places where land is more expensive, like downtown New York, that's where rent is exploding and appreciating faster in the tight urban areas."

As with so many things, it's also possible that millennials are to blame. Young people these days are moving to cities and starting families later. All those people living alone have little need for extra space, increasing demand for one-bedroom homes in some areas.

Those metro breakdowns include the full metro area — both the dense inner city and the more spacious suburban areas. However, if we just look only at the city itself, some cities still have the more traditional price structure with more expensive two-bedrooms.

In Atlanta and Chicago, two-bedrooms are still more expensive. And in those cities, two-bedrooms are actually increasing in price relative to one-bedrooms, suggesting that the trend is driven by geography within each city.

Two-bedroom rentals may be a great deal for people willing to live on the outskirts of the city, but in the cramped heart of the city, a cheap two-bedroom is hard to come by.

In New York, one-bedrooms are more expensive, as in the national trend, but it looks like two-bedrooms are catching up. In those extremely expensive areas, tenants may be even more eager to find a roommate to make rent affordable, making two-bedrooms valuable.

Popular American cities may have plenty of one-bedroom homes for those looking to live alone, but living in one of those homes isn't going to be cheap.